Tree-Pruning Essentials: Part Two

Pruning a tree to develop a strong, stable structure is the most important objective for the tree. Failure to develop a dominant leader results in multiple leaders, as shown here, and a weak branch structure. Photo courtesy of the author.

This is the second part of a two-part series on the essentials of tree pruning. The first part (“Tree-Pruning Essentials: Part One,” TCI Magazine, April 2020) involved types of pruning cuts, making the cut and expected responses from the tree. In part two, we will cover what to consider pruning in order to create a strong form, as well as the timing of making pruning cuts on trees.

What to prune

Pruning trees to develop a strong, stable structure is the most important objective for the tree. The significant goals to keep in mind are to focus on the development of a dominant leader, maintain appropriate clearance between the tree or any nearby objects to prevent conflicts and develop the desired appearance.

Choosing a dominant leader can be a challenge, but it is very important in developing a strong, sustainable tree. This can be done with three steps:

• Be sure to select the dominant stem in the center of the tree that is healthy and free of any defects.

• Identify any stems that may be competing with that central dominant stem, preventing a true central leader.

• Remove those competing stems and branches back to the trunk or subordinate them with reduction cuts.

This practice should be the guideline for any structural pruning, especially on young to medium-age trees. Be sure to maintain proper pruning doses during the trimming episode. When working on the tree, focus on the target branches as opposed to the entire tree. In other words, look for the largest limbs in the canopy that serve as the main scaffold branches, typically four to five major limbs. Then select the branches that need to be removed or reduced from those primary limbs to obtain the pruning goal, maintaining the appropriate amount of live foliage removed. Maintain a larger aspect ratio for strong branch unions.

A good central leader is key to a tree developing a strong, stable structure. Photo courtesy of the author.

Strong branch unions are those considered to have the proper branch-aspect ratio. Aspect ratio is the diameter of the branch relative to the diameter of the trunk, both measured immediately above the union. Branches with a small aspect ratio are very well attached to the trunk; those with a large aspect ratio separate more easily from the trunk.

The branch-aspect ratio can be made smaller by slowing the growth rate of the branch relative to the trunk or stem. The goal should be aspect ratios of 50% or less on permanent branch selections. Branches with a small aspect ratio of less than 50% are better attached to the trunk and better able to resist failure. In other words, maintain a low aspect ratio between trunk or stem and branch size of less than 2 to 1, meaning the size of remaining branches should be half that of the supporting trunk or stem. Try to establish branch attachments that do not have narrow angles and do not have included bark in the branch unions. Codominant stems and weak branch unions with included bark formation can be removed with careful pruning strategies to improve physical strength. Stems and branches with included bark are much more prone to splitting and decay, creating dangerous situations.

Aspect ratio is the diameter of the branch relative to the diameter of the trunk, both measured immediately above the union. At left is a good aspect ratio, while the example at right has a poor aspect ratio. Photo courtesy of the author.

Young trees

Pruning procedures vary greatly between a younger, newly established tree and a mature tree, but the biological principles are similar. However, goals change as the tree grows and develops into its mature size and form. For younger, smaller trees, minimize pruning until the tree is established; however, pruning at planting is an acceptable practice and is encouraged for proper development. The goals for newly planted trees and newly established trees are to create one dominant trunk system and to choose and establish the permanent, lower branches.

One of the first steps is to select a leader in the canopy. Often, we find there is a codominant stem contending with the top leader stem. If possible, select the strongest, straightest stem and remove any competitors. Allowing a codominant stem to grow will result in a weak arrangement more susceptible to splitting with high winds, especially after it grows in size. After choosing the appropriate leader, the central stem of the tree will form a strong, stable configuration.

Then, prune as needed on the young tree, continuing to establish strong branch unions with large aspect ratios and maintaining a dominant central form. The primary advantage of early pruning is making smaller wounds (due to smaller branches) and helping the wounds seal much faster. Selective removal and reduction of stems and branches early in a tree’s life creates a safer, stronger, more aesthetic structure as well. Of course, it is important to know the growth habit of the tree before proceeding. This procedure may not apply to clump-form or weeping-form trees or the creation of specialized habits such as pollarding or topiary.

Structural pruning can be done just about any time while the tree is young, during establishment and through medium age of maturity. According to research, pruning at planting time to improve structure provides no disadvantage. However, take time to analyze the tree carefully, selectively cutting to avoid excessive live-branch removal and disfigurement of the tree. Understand the form and growth habit of the tree species to get the desired results.

Allowing a codominant stem, as shown here, to grow will result in a weak arrangement more susceptible to splitting. Photo courtesy of the author.

Mature trees

As trees become larger and mature into properly selected locations, some pruning may be required to manage developing conflicts or to repair damage caused by storms, natural aging and pests. On established trees, focus pruning on reducing risk and enhancing appearance. The primary objective in pruning mature trees is reducing the largest branches, which are often the ones most likely to fail. Focus on reduction where necessary to improve stability, by reducing loads and improving clearance.

The strategy for pruning mature trees is to understand that the objectives include a focus on health, aesthetics, clearance and risk. First, examine the tree to determine what could affect health and appearance. This includes dead or dying twigs and branches, unnecessary sprouts and branches in decline. From a risk-mitigation perspective, review the canopy for crossing or rubbing branches, those branches with a large aspect ratio and codominant branches. Reducing the likelihood of failure is important.

Pruning early to develop a dominant leader creates a strong central form and a safer, stronger, more aesthetic structure. Photo courtesy of the author.

Also, remove unwanted epicormic and basal sprouts or water sprouts. Epicormic sprouts are branches that sprout from dormant buds on shoots that elongated in a previous period of growth. This type of growth is weakly attached, making it more prone to damage and more susceptible to pests. Basal sprouts are shoots that may arise from roots or adventitious buds around the root collar, and are not useful or helpful to the tree. On larger, mature trees, sprout-generated branches can grow from old, broken limbs and become an integral part of the canopy. Also, sprouting may be necessary on storm-damaged trees to replace lost limbs. Management strategies can be adapted to accommodate these characteristics of older or damaged trees.

After you complete the initial pruning, step back and check the tree for any unwanted growth from the crown that may influence appearance or other clearance issues. This phase of the process should be minimal, since a majority of the pruning has been completed. If possible, maintain the pruning dose to remove no more than 10% of foliage during the year. Live green tissue is of critical importance for energy and recovery. As the standards indicate, remove only what is necessary to achieve your objectives.

Before deciding to remove any branch, be sure the removal is really necessary and doesn’t compromise health or stability. Seriously consider the consequences before removing larger, structural branches that are 4 to 6 inches in diameter or greater. Branches of this size have a profound effect on long-term health and stability. If large limbs must be removed, consider progressive reduction to better facilitate recovery and reduce stress on the tree.

When to prune

There is much discussion and research on the best time to prune trees. Most of the time, pruning doesn’t occur until there is a problem. However, proper timing for pruning depends on tree health, environmental conditions, season and developmental age. Regardless of the need, always take into consideration the outcomes of the pruning action and what is best, long-term, for the tree. Prune trees when young to enhance growth and structure. Prune mature trees on an as-needed basis to ensure safety and to improve structure and necessary clearance.

You can prune to remove dead wood almost any time of the year. This will not have an impact on resources for the tree or pruning dose. The optimal time to prune green wood or live branches is in the late spring and early summer for the quickest, most effective recovery of pruning wounds. This is when the parenchyma cells are most active during the growing season and these are the cells most responsible for wound recovery. Defense systems (compartmentalization of decay in trees, or CODIT) produce boundary layers, or callus tissue, and wound wood develops and seals fastest on cuts made shortly before or early in the season of active growth.

However, trees may be pruned at just about any time of the year. Pruning in late winter or early spring, just before the new growth emerges, can be appropriate for many trees. This leaves wound tissue exposed for a shorter period of time before sealing begins. Also, with no leaves on the trees, branch structure is more visible, helping with the decision-making process on pruning cuts. Minimize any pruning in late summer or early fall, because that can promote a late flush of new growth more susceptible to cold damage and can delay dormancy on species such as elms and maples. Also, reconsider any pruning activity if the tree is suffering from environmental stress.

Always consider that any arboricultural practice should not spread pathogens in the process. Proper timing of pruning can reduce the spread of certain diseases. Dormant pruning, while trees are not actively growing, may be a good maintenance option on trees where pathogens such as oak wilt may be spread. Avoid pruning until late fall or dormancy, if disease is a problem. Spring or summer pruning increases chances for spread and infection of bacterial diseases such as fire blight. Prune crabapples, ornamental pears and hawthorns in late February through March if these diseases are an issue.

Trees are dynamic, living organisms that respond to outside stimuli, including pruning. Always consider the season and growth cycle before pruning and consider the physiological demands of the tree. Take into consideration the health of the tree. Review the internodal growth in twigs to determine its vigor. There exists a delicate balance of chemicals and carbon that allows the tree to thrive. Improper pruning practices cause a disruption of that balance. You can prune almost any time, but there is always an ideal time.

Conclusion

Trees are important assets that provide functional and aesthetic benefits. Often, we plant trees in less-than-favorable environments, such as urban areas, so they require help to survive these often-hostile conditions. The goal in any maintenance program should be to maximize the benefits of the tree while minimizing inputs required for it to survive. This is better for the tree owner, the tree and our environment.

Pruning is often a necessary activity, yet it can be devastating if done incorrectly. The best advice for any tree maintenance, including pruning, is never to let the situation exceed your skills. If you don’t know what you are doing when it comes to tree-maintenance activities, leave it alone. There are many resources available to the tree owner to assist with plant-health-care decisions. Be sure assistance comes from reputable, credible sources that are current with research and best-management practices. This is every professional arborist’s responsibility.

Lindsey Purcell is an urban-forestry specialist in the Department of Forestry and Natural Resources at Purdue University in West Lafayette, Indiana. This article is based on his presentation on the same subject at TCI EXPO 2019 in Pittsburgh, Pennsylvania, last fall. An audio recording of that presentation is below.

For additional information on tree pruning and response, visit the Purdue Education Store online.

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